sory i should have explained - live is on my mac, and the fast track pro is on my pc runing the VST's

so its down to the pc and chainer which im running to host the vsts but both the M-audio properties and chainer properties are the same: it wont let me adjust the latency without it snappling back to 256ms??

I know i can get away with 512 at the least but when i try and change it to cure the crackles, and press apply or OK it just defaults back to 256 again???
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If i'm right, you're attempting to change the buffer size from within Live while it's running? This means you've got the audio driver open at the time. In order to change the buffer you have to disable the audio in live (or use a different audio device (maybe another internal one), then you have to go to the Control Panel. Load up the Fast Track Pro icon and adjust the latency from within there. Hope that makes sense. I'm not on my PC but this is what I've had to do.

You have to change the latency when you are NOT running Live through the icon in your taskbar/system tray. Doing it through the Hardware settings in Ableton (or any other DAW) gets the snapback effect.

If the Fast Track Pro correctly works with your Windows PC, you should be able to play plugins with a buffersize of 128 samples. I had the FTP running on my Armada notebook (600mhz, 256mb ram, xp) really smoothly with latency of 128 samples. However, if I connected the same FTP to another faster Compaq notebook, I had glitches and crackles constantly. This was possibly caused by IRQ sharing conflicts

Also, keep in mind, if you use the SPDIF connection, you can't monitor your audio with zero latency settings.. You need to run the incoming audio signal through the software (Ableton) to be able to hear it. The zero latency monitoring option is only available for the analog inputs, which was the reason I sold it.